This invention relates to a novel method and apparatus for deflecting high current ion beams. More particularly, the invention concerns a method of retaining space-charge neutralization of the ion beam by preserving the electron cloud formed about the ion beam along the beam path.
In ion beam apparatus, such as accelerators and/or scanning apparatus, it is usual to produce a beam of ions from a suitable ion source and to control the movement of such ions along a predetermined path or trajectory through a vacuum environment, such as that provided by an evacuated cylinder or beam channel which is normally held at ground potential. It has been recognized, however, that such a beam channel cannot be perfectly evacuated. It has been suggested theoretically that electrons are formed, largely due to ionization, by collisons occuring between the positively charged ions and molecules of gas which exist within the vacuum chamber. Moreover, additional electrons are created upon impact of the ions against the target in the case of a scanning apparatus. Such electrons, due to their characteristic negative charge tend to be attracted to the positively charged ion beam.
Accordingly, the electrons have been found to form a cluster or cloud around the ion beam which tends to maintain the shape of the ion beam. Ordinarily, the positively charged ions exhibit an inherent tendency to diverge from each other in the absence of the electron cloud between the source and the target. However, the presence of electrons impart charge neutralization to the beam which enables deflection of relatively high current ion beams along a lengthy beam path without loss of resolution.
However, heretofore in the prior art, deflection plates oriented along the beam path to control the deflection of the ion beam, such as is necessary to scan a given target in a predetermined fashion, have been driven with alternating current signals, usually about a balanced ground. Thus, the positively charged ions are passed between a pair of parallel deflection plates which are alternately energized by a relatively high positive voltage level. It should be apparent that in such instances the positively charged plate, in the absence of other field influences, tends to attract negatively charged electrons. The same effect is observed when a positively charged probe is placed in the vacuum tube at a location proximate the ion beam path.
It has been recognized that the use of a positively charged deflection plate will tend to attract sufficient secondary electrons to degrade or destroy the electron cloud at least in a region proximate the deflection plates, if not throughout most the length of the beam path. This, in turn, has led to limitations upon the magnitude of high current ion beams which could be successfully controlled by the usual deflection techniques within a scanning apparatus.
Consequently, there is a need in the art for means for eliminating the destruction of the electron cloud in ion beam systems whereby deflection of high current ion beams can be accomplished.